In a wireless hardware of a wireless system, the reduction of power consumption is an important factor. In particular, in a wireless communication system, as examples of application, for which a power-saving mechanism is needed, there are an active electronic tag system, a sensor network system, etc. For sensor nodes in the active electronic tag or in the sensor network, portability and easy and flexible installation are important features. Normally, it is a battery type node incorporated with a small-size battery.
The applications such as the active electronic tag system or the sensor network system are characterized by low traffic. In the active electronic tag system, small-size data including the active electronic tag's own ID (identification information) is transmitted. Also, in the sensor network system, the sensor node performs intermittent transmission of small-size sensing data in most cases.
In the active electronic tag system, the exchange of information is conducted in such manner that data transmission from the electronic tag is received by a reader. In normal case, the active electronic tag does not have any relation with the other active electronic tag, and for the purpose of receiving information from the active electronic tag transmitted at random, the reader must have its receiver always ready for starting. Also, in the active electronic tag system, each of the active electronic tags independently behaves, and data collision may occur due to simultaneous transmission of a plurality of active electronic tags.
In order to cope with the diversification of the mode of the use of communication in recent years, it is important that the communication must be performed bi-directionally. In the active electronic tag system, however, uni-directional communication is performed from the electronic tag to the reader. Even when a node, which serves as reader/writer in one unit, may be used with the electronic tag, and it is difficult to reduce the power consumption because the receiver is always in starting state, and there are also problems in access control (collision avoidance control).
As described above, the reduction of power consumption in the wireless equipment and system is an important factor, and several methods have been proposed to solve the problems in the past. On of the methods is that, in low traffic network such as a sensor network, when no data is transmitted and received and a wireless circuit (wireless communication function) is not used, the wireless circuit is turned off (to a sleeping state). In this type of intermittent communication, when the wireless circuit is turned off at both the transmitting side and the receiving side, it is important how synchronization is performed between the transmitting node and the receiving node in the timing of transmission and receiving.
One method for solving the problem in the intermittent bi-directional communication is a method to adopt ZigBee (registered trademark) using MAC layer as defined in IEEE 802.15.4. According to IEEE 802.15.4, when a synchronous node called “beacon mode” is used, each node is synchronized by a beacon transmitted from a coordinator acting as a control unit. Also, in ZigBee, by defining a super-frame as shown in FIG. 20 and by increasing beacon interval longer than the super-frame period, an idle period or an inactive period can be defined. All of the synchronized nodes can be made in the sleeping state during this idle period. When data transmission is carried out according to synchronization schedule by the super-frame, data can be received at each of the nodes.
According to IEEE 802.15.4, the super-frame period is divided to 16 slots (timeslots). When the data are transmitted, each node selects a slot and transmits the data by using CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) within the slot and the change of data transmission can be maintained in fair and adequate manner between the nodes even in case the number of the nodes in the system has increased.
In the latter half of the super-frame period, a guaranteed time slot (GTS) is defined. This GTS is a period where it is guaranteed that the coordinator can preferentially use the slot to a specific node. Therefore, the communication at this GTS is not allowed to the node other than the specific node, to which the use of the communication by GTS is permitted. As shown in FIG. 20, the length of GTS is assigned when the node negotiates with the coordinator. As a result, communication can be performed by avoiding the collision of the transmission data.
However, according to IEEE 802.15.4, the slot to be used at GTS within the super-frame is limited. When the number of nodes to request slot reservation is increased, it is difficult that all nodes requesting the slot reservation receives the assignment of GTS in a single super-frame period. In this case, the node, which could not receive the slot reservation, must wait until the next super-frame period. So far as it is configured that the assignment of GTS is received from the coordinator and the coordinator is notified that GTS has been assigned to a node, which is present within the range where electric wave can reach, network topology is limited to star type. The star type topology lacks flexibility in its configuration, and it has such defect that it is used under the assumption of topology when a mobile node performs communication.
As described above, in order that the slot reservation is executed according to IEEE 802.15.4, there is a limitation to the number of slots, which can be used for the slot reservation in a single super-frame period. Also, according to IEEE 802.15.4, a coordinator for the management of the slot reservation must be present. As a result, there is a restriction in the network configuration (restriction on the network topology).
Unlike the case of the communication at the super-frame of ZigBee, a technique to reduce power consumption in beacon-less solution without system synchronization by beacon is disclosed in the Patent Document 1 as given below (see FIG. 21). According to the technique disclosed in this Patent Document 1, when it is set to TL<<TPL where TPL is a monitoring cycle and TL is a monitoring period, each receiving node is started for a certain fixed time periodically and monitors air interface.
On the other hand, a transmission source node (source node) issues a wakeup signal (WU) during this TPL period. As a result, each of the receiving nodes grasps that the data corresponding to the wakeup signal is transmitted during TL, which is own receiving period of the node itself. When information relating to the time to start the transmission (time pointer) is included in the wakeup signal, it is possible to grasp the starting time (the time to start data transmission).
Further, in case destination address is included in the wakeup signal as the information to be transmitted on the wakeup signal, if only the node, which has received the wakeup signal, starts the receiving unit, the starting time of the other node can be decreased. Also, it is possible to shorten the transmission time to transmit the wakeup signal, which is needed during the period of the monitoring cycle TPL, if sampling schedule of each node is held as table information by mutually giving and taking the time to terminate the communication and the time to start the communication in the next monitoring period of the node at each communication.
In the system disclosed in the Patent Document 1, the transmitted data is preferentially received when the starting time of each of the receivers of a plurality of nodes is equalized. Further, the starting time to start data transmission can be designated to the time as requested by the node by sending the wakeup signal, and it is also configured that the data transmission is preferentially controlled.
However, in the system disclosed in the Patent Document 1, the wakeup signal must be transmitted during the period of the monitoring cycle TPL until the table information is obtained. As suggested in the Patent Document 1, not only that the power is consumed at the transmission source node but also wireless channel is occupied for a considerable period of the time cycle. As a result, the transmission of other wakeup signal by the other node may be hindered or collision may occur in the other transmission currently going on. In particular, when communication is performed by a multiple of unspecific wireless nodes while these are moving, it is considered that the chance where the giving and the taking of the wakeup signal are needed until the completion of the first table configuration.
Therefore, in the system as disclosed in the Patent Document 1 as given above, for the purpose of executing preferential control of the data transmission, it is necessary to have long channel occupying period and long setting time until the data transmission is started.
[Patent Document 1] Japanese Patent Application Publication No. 2006-148906
In order to reduce power consumption in the wireless communication system, data transmission techniques by periodical data transmission (communication by radio frequency identification (RFID) or the use of active electronic tag or by ALOHA system using periodical broadcasting) are effective as the simplified data transmitting means to a plurality of nodes in wireless short distance communication without specifying the correspondent node (CN). In particular, the data transmission technique by periodical data transmission is effective for the communication between mobile nodes (MN). The effectiveness of this technique lies in that it is possible to decrease the number of messages to be exchanged by performing the communication according to a simplified protocol, to reduce data transmitting/receiving time by decreasing the amount of the transmission data, and to speed up the processing of power saving and to quickly start and terminate data exchange.
In case short data (data of small data amount) is transmitted periodically, there is a limitation in the amount of information to be exchanged between two nodes by a single data transmission and receiving. In this case, the amount of information to be transmitted by many of data transmission and receiving can be increased, but wireless communication must be guaranteed, which causes no collision in the data transmission. However, the simplified protocol is not configured to avoid the possibility of the collision of data transmission, and reliable maintenance of the wireless communication route cannot be guaranteed.
Also, according to IEEE 802.15.4, the guarantee of the slot by GTS is defined in the super-frame period, while a problem arises in that there is a limit (upper limit) to the number of slots to be used for slot reservation within a single super-frame period. Also, a coordinator for the management of the slot reservation is needed, and there is also a problem that there may be restriction to the configuration of the network. The network topology restricted by IEEE 802.15.4 is not suitable for the case where data transmission and receiving are performed by P2P (Peer to Peer) between MNs.
According to the system disclosed in the Patent Document 1, it is possible to guarantee the data transmission and receiving by uniformly synchronizing the starting time of the receiver of peripheral nodes by defining the wakeup signal, while there is a problem in that the channel occupying period (a period from the transmission of the wakeup signal to the transmission of data) is long, during which the channel is occupied by the wakeup signal transmitted before the starting of the data transmission by a certain node. As a result, when the transmission nodes increase, the chances of the occupying of the channel are increased, and this makes it difficult to guarantee the data transmission. When the attention is given on the data transmission and receiving between MNs, the period from the transmission of the wakeup signal to the transmission of data is long, and when MN transmits the data while the channel is occupied by the wakeup signal, it may move out of the propagation range (i.e. the communication between MNs may not be performed).
In the technique disclosed in the Patent Document 1 as given above, the wakeup signal must be transmitted within the period of the monitoring cycle TPL until the table information is obtained. As suggested by the Patent Document 1, the power of the transmission source node is consumed during this period and the wireless channel is occupied during a considerable length of the period of time cycle. As a result, the transmission of the other wakeup signal by the other node may be hindered or collision may occur in the other transmission, which is currently going on.
In particular, when a multiple of unspecified wireless nodes perform communication while moving, the cases where the giving and the taking of the data for long period using the wakeup signal WU may be increased until the first table configuration, and this means that power may be consumed uselessly. Similarly, in the case where transmission is performed to a multiple of unspecified nodes (broadcast or multi-cast), the wakeup signals must be notified during the monitoring period of each node, and power may be consumed uselessly. In the system where information of the data to be transmitted is to be notified by the wakeup signal before the actual outputting of data and where the timing of transmission and receiving is to be synchronized with the receiving node, the wakeup signals must be relatively large with respect to the data when small data is given and taken not very frequently, and this may make the communication inefficient.
In the technique disclosed in the Patent Document 1, when sampling schedule of each node is held as table information, the transmitting node transmits the wakeup signal only during the monitoring period of the receiving node (i.e. the correspondent node) and it notifies the timing to start its own transmission. Unlike the case where the wakeup signal is transmitted during the monitoring cycle TPL, the node to recognize the data transmission period of the transmitting node is only the receiving node, which is the correspondent node. Thus, even when the other transmitting node may have the possibility to transmit the data at the same time to the other receiving node, it is not possible to detect it. As a result, the probability of signal collision may increase, and it may be difficult to have adequate access control (collision avoidance control) between the transmitting nodes.